Passive robots intended for collaboration with a human operator, which are sometimes called “cobots,” move in response to a force that a user manually applies to its end point. Limits or constraints placed on the end point position determine the end point trajectory, while the energy to move the end point along the trajectory is supplied by the user. These limits collectively define “virtual surfaces” which separate a workspace into free regions, in which a user may freely move the end point of the cobot, and restricted regions, in which the user cannot freely move the end point.
Although cobots may utilize motors, the motors are not used to move the end point along the trajectory. Unlike motor actuated joints of conventional robots, the joints of a cobot include nonholonomic elements. Revolute joints in cobots are commonly formed using continuously variable transmissions as a nonholonomic element. A continuously variable transmission (CVT) is one that can vary its transmission ratio over a continuous, predefined range of values. A continuously variable transmission that can vary its transmission ratio over a continuous range of values including zero and including reversal (negative values) is sometimes referred to as an infinitely variable transmission.
Examples of cobots and CVTs used by them are disclosed in the following publications, all of which are incorporated herein by reference: Book, W., R. Charles, et al. (1996). The concept and implementation of a passive trajectory enhancing robot. International Mechanical Engineering Congress and Exposition, ASME; Colgate, J. and J. Brown (1994). Factors Affecting the Z-Width of a Haptic Display. IEEE International Conference on Robotics & Automation, San Diego, Calif.; James E. Colgate, Michael A. Peshkin, Witaya Wannasuphoprasit, Nonholonomic Haptic Display, Proceedings of the IEEE 1996 International Conference on Robotics and Automation, Philadelphia; Eric L. Faulring, J. Edward Colgate and Michael A. Peshkin, (2004) A High Performance 6-DOF Haptic Cobot. IEEE International Conference on Robotics and Automation; Eric Faulring.; J. Edward Colgate; Michael A. Peshkin (2005), High Performance Cobotics. IEEE 9th International Conference on Rehabilitation Robotics, Jun. 28, 2005; Gillespie, R. B.; Colgate, J. E.; Peshkin, M. A., (2001), A general framework for cobot control, IEEE Transactions on Robotics and Automation, 17(4) p. 391, August 2001; Carl Moore, Michael A. Peshkin, J. Edward Colgate, (2003), Cobot Implementation of Virtual Paths and 3D Virtual Surfaces, IEEE Transactions on Robotics and Automation, 19(2), p. 347-351, April 2003; and Michael A. Peshkin, J. Edward Colgate, Witaya Wannasuphoprasit, Carl Moore, Brent Gillespie, (2001), Cobot architecture, IEEE Transactions on Robotics and Automation, 17(4), p. 377, August 2001. Cobots and cobot transmissions are also disclosed in U.S. Pat. Nos. 6,686,911, 5,952,796, and 5,923,139, which are also incorporated herein by reference.
Conventional electromechanical systems often do not use continuously variable transmissions and are limited to a fixed gear ratio. Subsequently the combination of a specific actuator and a single gear ratio may not be able to output a target maximum effort at low speed, since the fixed gearing causes the actuator to operate at a power inefficient speed. Thus a much larger actuator and a larger power supply must be chosen to satisfy the maximum speed and maximum effort requirements, given the fixed gear ratio, causing the system to operate at power inefficient speeds. This larger actuator likely has the capacity to deliver more power than is needed at certain speeds.